This invention relates to the production of a thermoplastic polymer structure having comparatively high density skin layers and a comparatively low density core layer. In particular, it relates to a method and apparatus for forming a polystyrene sheet structure having comparatively high density skin layers and a comparatively low density core layer.
It is known to produce polystyrene foam structures which have a comparatively low core density in relation to comparatively high skin layer densities. See, for example, U.S. Pat. Nos. 3,864,444 and 3,299,192.
U.S. Pat. No. 3,299,192 (Lux) states that the rigidity, liquid handling, and thermal insulation capability of foamed plastic pipe is enhanced by quench chilling the internal and external walls of a tube within a short time after it immerges from an extrusion die. The patent notes that such chilling produces a impervious and non-porous internal and external skin on the pipe. The patent teaches that the inner skin might be made to five times as thick as the outer skin for purposes of carrying liquids, such as water. The disclosure suggests coolant temperatures of 0.degree. to 80.degree. F. The examples recite the use of 70.degree. F. air as a coolant, blasted from a nozzle upon the extrudate surfaces. Such chilling produces a pipe having a core representing 50-94% of the thickness of the pipe, the interior and exterior skins representing the balance of the pipe thickness.
It is an object of the present invention to present a process for the production of foam polymer sheet having high density skin layers and a low density core.
A further object of the invention is to present apparatus capable of forming such a polymeric structure.